Capacity control system for compressor units



Nov. 25, 1969 F. BAUER 3,480,199

CAPACITY CONTROL SYSTEM FOR COMPRESSOR UNITS Filed NOV. 24, 1967 FIG]FIG?

V77Z OF Fnedmck Buuel' United States Patent US. Cl. 2304 5 ClaimsABSTRACT OF THE DISCLOSURE A capacity control system for multi-stagecompressor units comprising control means for altering the delivery ofthe first-stage compressor of the unit and additional control means foraltering the delivery of one of the subsequent compression stages.

BACKGROUND OF THE INVENTION This invention relates to a capacity controlsystem for a compressor unit having a first-stage compressor followed byat least one subsequent compression stage, and connecting line betweenthe compression stages.

Compressor units, particulary of the type comprising a plurality ofcompression stages, are generally required to provide adaptation of thequantity delivered to the actual needs of the consuming unit or to thevolume of gas produced so as to keep the suction, the final or anintermediate pressure at a constant level. The control systems used forthis purpose are supposed to operate at low cost and possibly withoutany power losses and to offer the required degree of operational safety.Conventional systems wherein all compression stages are provided withindividual regulators, such as for example, with lifting devices for thesuction valves, operate at negligible power losses. However thestructural expense involved is comparatively high. Consequently, in manycases only the first compression stage of a multi-stage compressor unitis subject to control whereas the higher compression stages remainuncontrolled, their deliveries diminishing only as a result of acorresponding decrease of the intermediate pressures.

If for example, the final pressure of a mnlti-stage compressor unit isto be kept at a constant level by controlling the delivery of the firstcompression stage only, the resulting lower periods being frequently ofgreat length due to the slow alteration of individual intermediatepressures. Consequently, the compressor assembly will respond to eachcontrol pulse by altering the delivery of the last stage after aconsiderable delay only. Therefore, unless the alterations are veryslow, oscillations are liable to occur running through the entirecontrol circuit. In many cases stability may be maintained by the use ofregulators featuring a specific time behaviour. This is, however,possible only at the expense of the quality of control, such operatingconditions as the control system is meant to ensure, cannot as a rule,be maintained for any length of time within the desired limits.

SUMMARY OF THE INVENTION It is the object of the invention to reduceobjectionable lower periods in connection with the capacity control ofcompressor units and to provide a control system capable of adapting thecompressor unit to given operating conditions with the least possibledelay at low structural expense.

The invention provides for a capacity control system for a compressorunit having a first-stage compressor fol- Patented Nov. 25, 1969 lowedby at least one subsequent compression stage, and connecting linesbetween the compression stages, comprising control means for alteringthe delivery of the first-stage compressor and additional control meansfor altering the delivery of at least one of the subsequent compressionstages, the said first control means being adjusted as required bycontrol pulses, said additional control means being temporarily adjustedfor a brief period of time only dependent on a change in a physicalcondition prevailing in the connecting line between two precedingcompression stages thereby altering the delivery of the said subsequentcompression stage and reducing the time required by a control pulsereleased to the first-stage compressor controlling the means foraltering the delivery of the first-stage compressor, for its passagethrough the entire compressor unit.

By this simple measure any control intervention with the first-stagecompressor is immediately transmitted to the end of the control range,for example, to the last compression stage so that lower periods arelargely eliminated and overriding is avoided. Consequently, the controlsystem according to the invention assures permanent maintenance, with ahigh degree of precision, of the operating condition required at the endof the control range, such as for example, of the final pressure of thecompressor unit, within narrow limits.

Preferably the last compression stage of the compressor unit is providedwith said additional control means for altering the quantity delivered.The additional control means may be controlled as a function of aphysical condition, preferably of pressure, in the connecting linebetween the first-stage compressor and the adjacent compression stage.All or part of the compression stages may consist of piston-typecompressors. Moreover, the temporarily acting additional control meansfor altering the delivery of one of the subsequent compression stagesmay be operated according to any control system. Since in normaloperation the compressor unit is controlled by means of the controlmeans acting upon the first-stage compressor only and the additionalcontrol means are in operation only temporarily, a power loss-involvingmethod can be used for the purpose without perceptibly impairing theefficiency of the whole system.

Therefore, the additional temporary control means in a simple manner maycomprise a control line branching off the delivery side of the saidsubsequent compression stage, a throttle valve inserted in the controlline for the purpose of altering its cross-sectional area, and a controldevice liable to be influenced by a physical condition prevailing in theconnecting line between two compression stages and controlling the saidthrottle valve. The control line may terminate into the atmosphere orreturn at any convenient point to the connecting line between twocompression stages, preferably to the connecting line by which theassociated control device is impinged upon, such as for example, to theconnecting line between two compression stages, thereby forming aby-pass.

BRIEF DESCRIPTION OF THE DRAWING Further optional details and advantagesof the invention will appear from the following description of severalembodiments with reference to the accompanying drawing in which:

FIGURE 1 is a schematic view of a three-stage compressor unit for thepurpose of explaining the control system according to the invention, and

FIGURE 2 is a schematic illustration of a compressor unit comprisingfour compression stages and equipped with a control system according tothe invention.

3 DESCRIPTION OF THE PREFERRED EMBODIMENTS The individual compressionstages are designated by reference numbers I to IV, stage I defining thefirst-stage compressor and the remaining stages II to IV the subsequentcompression stages following the first-stage compressor. A suction pipe1 terminates in the first-stage compressor I, connecting lines 2 areprovided between individual compressor stages and the consumption pipe 3emerges from the last compression stage.

As illustrated schematically in FIG. 1, a control pulse is released tothe first-stage compressor I via a control line 4, the effect of thesaid pulse being a decrease of the quantity delivered by the first-stagecompressor I into the adjacent connecting line 2. As a result, theintermediate pressure in the connecting line 2 drops and so does thesuction pressure of the second compression stage II, thereby decreasingalso the quantity delivered by that stage accordingly. Without anadditional control of the higher compression stages the suction pressureof the third compression stage II will then drop with a retardation ofthe second order and so will the quantity delivered by the compressionstage II, and finally the control pulse released to the first-stagecompressor I will become effective in the consumption line 3 with aretardation of the third order.

In order to shorten the time required by the control pulse to passthrough the compressor unit, the quantity delivered by the compressionstage III is temporarily altered in accordance with the control pulse asa function of the intermediate pressure prevailing in the connectingline 2 emerging from the first-stage compressor I. For that purpose, acontrol device R is provided which is connected to the connecting line 2via a line 5 and releases a control pulse to the compression stage IIIvia a control line 6. The retardation due to the passage of the controlpulse through the compressor unit in the compression stage II is thuseliminated so that the down time of the compressor unit iscorrespondingly reduced. The reaction velocity of the control system isthereby increased to a considerable extent. This feature is particularlyimportant for compressor units comprising a plurality of compressionstages.

According to the embodiment of the invention illustrated in FIG. 2, thefirst-stage compressor 1, such as a double-acting piston-typecompressor, is provided with a lifting mechanism 7 for the suctionvalves on each side of the cylinder, which are controlled in accordancewith the stroke rhythm by means of a main control device R via a controlline 4. Via line 8 the control device R is impinged upon by the finalpressure of the compressor unit prevailing in a pressure tank 9interpolated between the last compression stage IV and the consumptionline 3, and serves to keep the said final pressure at a constant level.The control device R is fed via a feed pipe 10. The higher compressionstages II to IV are by-passed by a by-pass line 11 emerging from thedelivery pipe 12 of the last compression stage IV terminating in thepressure tank 9, the said by-pass line 11 terminating in the connectingline 2 between the first stage compressor I and the compression stageII. An intermediate cooler 13 and a liquid draining device 14 areinterpolated in each of the connecting lines 2 between individualcompression stages and in the delivery pipe 12. The by-pass line 11 iscontrolled by means of an infinitely variable throttle valve 15 actuatedvia a control line 16 by means of a control device R which is in turncontrolled via line 5 as a function of the intermediate pressure betweenthe compression stages I and II. The control device R and the throttlevalve 15 are fed via a feed line 17 having incorporated an adjustablethrottle valve 18.

The control device R may for example, be a diaphragm governor comprising-a retarding throttle 19 and altering the pressure in the control line16 when the intermediate pressure in the connecting line 2 drops, as aresult of which the throttle valve 15 opens, allowing the compressedmedium to flow from the delivery pipe 12 via the bypass line 11 backinto the connecting line 2 between the compression stages I and II. Atthe same time, the pressure difference acting upon the control device Ris reduced by means of the retarding throttle 19, so that the formerslowly returns to its original position, thereby closing the throttlevalve 15. By means of the retarding throttle 19 the closing velocity canbe adapted to compensate the lack of reactivity of the highercompression stages II to IV in such a manner that no oscillations willoccur in the pressure tank 9.

The diaphragm governor R has an initial tendency to keep theintermediate pressure between the compression stages I and II at anapproximately constant level, as a result of which precisely the excessquantity of the pressure medium is blown from the delivery pipe 12 intothe connection line 2 forming the suction pipe of the compression stageII and causes a slow transition from the by-pass control to thenondissipative capacity control by means of reduced intermediatepressures, without influencing the final pressure in the process. Thusthe reaction velocity of the compressor unit control system issubstantially increased by means of the additional temporary controlmechanism. Since the by-pass control is operative only temporarily forbrief intervals following sudden major changes in consumption, theresulting loss of power is negligible.

The control system according to the invention is generally suitable foruse in conjunction with any compressor unit control involving theproblem of influencing the physical condition at one or several pointsof the compressor unit by means of a control pulse released to thefirst-stage compressor. In any case, the reaction velocity of thecontrol system is considerably increased by the reduction of the timerequired for the passage of the control pulse through the entirecompressor unit, so that objectionable oscillations and overriding areavoided and maintenance of the desired operational condition is assuredwithin narrow limits.

I claim:

1. In combination with a compound compressor unit having a first-stagecompressor followed by at least two subsequent compression stagesconnected by connecting lines, a capacity control system comprisingcontrol means for the first-stage compressor and additional controlmeans for at least one of the subsequent compression stages, the firstcontrol means being controlled by control signals, a control device forthe additional control means and initiated by a change in a physicalcondition prevailing in the connecting line between two precedingcompression stages, and adjustable regulator means for limiting theworking time of the control device, the said additional control meansbeing temporarily in working condition for a brief period of time only,altering the delivery of the said subsequent compression stage andreducing the time required for the passage through the entire compressorunit of a control signal released to the first-stage compressor.

2. In combination with a compound compressor unit a capacity controlsystem according to claim 1, in which the compressor unit comprises afirst-stage compressor and at least two subsequent compression stages,the said additional control means controlling the last compressionstage.

3. In combination with a compound compressor unit a capacity controlsystem according to claim 1, in which the compressor unit comprises afirst-stage compressor and at least two subsequent compression stages,the said additional control means controlling the last compressionstage, being controlled as a function of a physical condition prevailingin the connecting line between the firststage compressor and theadjacent compression stage.

4. In combination with a compound compressor unit a capacity controlsystem according to claim 1, in which the additional control means foraltering the delivery of at least one of the subsequent compressionstages comprising a control line branching off the delivery side of thecontrolled subsequent compression stage, a throttle =-valve beingprovided inserted in the control line for the pur' pose of altering itscross-sectional area and controlled by the said control device liable tobe influenced by a physical condition prevailing in the connecting linebetween two compression stages.

5. In combination with a compound compressor unit a capacity controlsystem according to claim 1, in which the additional control means foraltering the delivery of at least one of the subsequent compressionstages comprising a control line branching off the delivery side of thecontrolled subsequent compression stage, a throttle valve being providedinserted in the control line for the purpose of altering itscross-sectional area and controlled by the said control device liable tobe influenced by a physical condition prevailing in the connecting linebetween two compression stages, the control line forming a by-passconnecting the delivery side of the controlled subsequent compressionstage with the connecting line between two preceding compression stages.

References Cited UNITED STATES PATENTS 7 858,470 7/1907 Richards 230-22XR 1,759,617 5/1930 Hoerbiger 23022 1,863,845 6/1932 Halvorsen 230-26ROBERT M. WALKER, Primary Examiner US. Cl. X.R. 23022

